Light emitting device for AC power operation

ABSTRACT

Disclosed is an improved light-emitting device for an AC power operation. A conventional light emitting device employs an AC light-emitting diode having arrays of light emitting cells connected in reverse parallel. The arrays in the prior art alternately repeat on/off in response to a phase change of an AC power source, resulting in short light emission time during a ½ cycle and the occurrence of a flicker effect. An AC light-emitting device according to the present invention employs a variety of means by which light emission time is prolonged during a ½ cycle in response to a phase change of an AC power source and a flicker effect can be reduced. For example, the means may be switching blocks respectively connected to nodes between the light emitting cells, switching blocks connected to a plurality of arrays, or a delay phosphor. Further, there is provided an AC light-emitting device, wherein a plurality of arrays having the different numbers of light emitting cells are employed to increase light emission time and to reduce a flicker effect.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/994,308, filed on Dec. 28, 2007, which is the U.S. national stageentry of International Application No. PCT/KR2006/001726, filed on May9, 2006, and claims priority from and the benefit of Korean PatentApplication No. 10-2005-0056175, filed on Jun. 28, 2005, Korean PatentApplication No. 10-2005-0104952, filed on Nov. 3, 2005, Korean PatentApplication No. 10-2005-0126872, filed on Dec. 21, 2005, Korean PatentApplication No. 10-2005-0126873, filed on Dec. 21, 2005, Korean PatentApplication No. 10-2005-0126904, filed on Dec. 21, 2005, and KoreanPatent Application No. 10-2006-0013322, filed on Feb. 11, 2006, whichare hereby incorporated by reference for all purposes as if fully setforth herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a light emitting device, and moreparticularly, to a light-emitting device for an AC power operation,which has an array of light emitting cells connected in series.

2. Discussion of the Background

A light emitting diode (LED) is an electroluminescence device having astructure in which an N-type semiconductor and a P-type semiconductorare joined together, and emits light through recombination of electronsand holes. Such an LED has been widely used for a display and abacklight. Further, since the LED has less electric power consumptionand a longer service life as compared with conventional light bulbs orfluorescent lamps, its application area has expanded to the use thereoffor general illumination while substituting the conventionalincandescent bulbs and fluorescent lamps.

The LED repeats on/off in accordance with the direction of a currentunder an AC power source. Thus, if the LED is used while being connecteddirectly to the AC power source, there is a problem in that it does notcontinuously emit light and is easily damaged by reverse currents.

To solve such a problem of the LED, an LED that can be used while beingconnected directly to a high voltage AC power source is proposed inInternational Publication No. WO 2004/023568A1 entitled “LIGHT-EMITTINGDEVICE HAVING LIGHT-EMITTING ELEMENTS” by SAKAI et al.

According to the disclosure of WO 2004/023568A1, LEDs aretwo-dimensionally connected in series on an insulative substrate such asa sapphire substrate to form LED arrays. Two LED arrays are connected inreverse parallel on the sapphire substrate. As a result, there isprovided a single chip light emitting device that can be driven by meansof an AC power supply.

FIGS. 1 and 2 are schematic circuit diagrams illustrating a conventionallight emitting device having light emitting cells connected in series;FIG. 3 is a schematic graph illustrating a driving voltage and a currentwith time in the conventional light emitting device; and FIG. 4 is aschematic graph illustrating a driving voltage and a light emissionamount of the conventional light emitting device.

Referring to FIGS. 1 and 2, light emitting cells C₁ to C_(n) areconnected in series to constitute an array. At least two arrays areprovided within a single chip 15 in FIG. 2, and these arrays areconnected in reverse parallel to each other. Meanwhile, an AC voltagepower source 10 in FIG. 2 is connected to both ends of the arrays. Asshown in FIG. 2, an external resistor R1 is connected between the ACpower source 10 and the LED 15.

The light emitting cells C₁ to C_(n) of the array are operated for a ½cycle of the AC voltage power source and the other array connected inreverse parallel to the array is operated for the other ½ cycle thereof.Accordingly, the arrays are alternately operated by means of the ACvoltage power source.

However, the light emitting cells connected in series are simultaneouslyturned on or off by mean of an AC voltage. Thus, when the AC voltage hasa value larger than the sum of threshold voltages of the light emittingcells, a current begins to flow through the light emitting cells. Thatis, the light emitting cells simultaneously begin to be turned on whenthe AC voltage exceeds the sum of the threshold voltages, and they aresimultaneously turned off in a case where the AC voltage is less thanthe sum of the threshold voltages.

Referring to FIG. 3, before an AC voltage exceeds a predetermined value,the light emitting cells are not turned on and a current does not flowtherethrough. Meanwhile, when a certain period of time lapses and the ACvoltage exceeds the predetermined value, a current is begins to flowthrough the array of the light emitting cells. Meanwhile, when the timeis at T/4 while the AC voltage is more increased, the current has amaximum value and is then decreased. On the other hand, if the ACvoltage is less than the predetermined value, the light emitting cellsare turned off and a current does not flow therethrough. Thus, a timeduring which a current flows through the light emitting cells isrelatively shorter than T/2.

Referring to FIGS. 4 (a) and (b), the light emitting cells emit lightwhen a predetermined current flows through them. Thus, an effective timeduring which the light emitting cells are driven to emit light becomesshorter than a time during which a current flows through the lightemitting cells.

As the effective time during which light is emitted becomes short, lightoutput is decreased. And thus, a high peak value of a driving voltagemay be needed to increase the effective time. However, in this case,power consumption in the external resistor R1 is increased, and acurrent is also increased according to the increase in the drivingvoltage. The increase in a current leads to increase in the junctiontemperature of the light emitting cells, and the increase in thejunction temperature reduces the light emitting efficiency of the lightemitting cells.

Further, since the light emitting cells are operated only when thevoltage of the AC power source exceeds the sum of the threshold voltagesof the light emitting cells within the array, the light emitting cellsare operated in a rate slower than a phase change rate of the AC powersource. Accordingly, uniform light is not continuously emitted on asubstrate and a flicker effect occurs. Such a flicker effect remarkablyappears when an object moving at a certain distance from a light sourceis viewed. Even though the effect is not observed with the naked eye, itmay cause eye fatigue if the light emitting cells are used forillumination for a long is period of time.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a light-emitting devicefor an AC power operation, wherein a flicker effect can be prevented orreduced, and a time during which light is emitted is increasedcorresponding to a phase change in an AC voltage.

Another object of the present invention is to provide a light emittingdevice, wherein an operation can be made with a lower current ascompared with a prior art, thereby improving light emitting efficiency.

To achieve these objects, the present invention provides alight-emitting device for an AC power operation, the device having anarray of light emitting cells connected in series. An aspect of thepresent invention provides a light emitting device, wherein lightemitting cells within arrays are sequentially turned on and off. Thelight emitting device according to the aspect of the present inventioncomprises a light emitting diode (LED) chip having an array of lightemitting cells connected in series; and switching blocks respectivelyconnected to nodes between the light emitting cells. The light emittingcells are sequentially turned on and off by the switching blocks whenthe array is connected to and driven by an AC voltage power source.Since the light emitting cells are sequentially turned on and off, aneffective time during which the light emitting cells emit light can beincreased as whole.

The switching blocks may be connected to source and ground terminals ofthe array. At this time, an n-th switching block may short the groundterminal and a node to which the n-th switching block is connected whena voltage difference V_(ac) between the source and ground terminals isin a range of (Predetermined Voltage×n) to (PredeterminedVoltage×(n+1)), and may open the node from the ground terminal if thevoltage difference V_(ac) is greater than (Predetermined Voltage×(n+1)).Accordingly, when an AC voltage is increased, the switching blockssequentially repeat the short-circuiting and the opening so that thelight emitting cells can be sequentially turned on. When the AC voltageis decreased, the light emitting cells are sequentially turned off.

The predetermined voltage may be a forward voltage of the light emittingcells at a reference current. Accordingly, a current flowing through thelight emitting cells can be constantly maintained to be a currentapproximate to the reference current. Thus, the light emittingefficiency of the light emitting cells can be improved by adjusting thereference current. For example, the reference current may be 15 to 25mA.

The light emitting cells may be turned off in an order reverse to theorder in which the light emitting cells are turned on. Alternatively,they may be turned off in the order in which the light emitting cellsare turned on.

Each of the light emitting cells may comprise an N-type semiconductorlayer, a P-type semiconductor layer and an active layer interposedtherebetween. Further, the semiconductor layers may be made of a GaNbased semiconductor.

Meanwhile, the LED chip may further comprise another array of lightemitting cells connected in series, in addition to the array of thelight emitting cells connected in series. These arrays are connected inreverse parallel to each other. Switching blocks may be connected tonodes of the light emitting cells within the other array of the lightemitting cells connected in series, respectively. At this time, theswitching blocks connected to nodes of the light emitting cells withinthe array of the light emitting cells connected in series may beconnected to the nodes of the light emitting cells within the otherarray, respectively.

Another aspect of the present invention provides a light-emitting devicefor an AC power operation, the device comprising a plurality of arraysthat have different numbers of light emitting cells connected in seriesand are connected in parallel to one another. The AC light-emittingdevice according to this aspect of the present invention includes asubstrate. A plurality of first arrays are positioned on the substrate.The first arrays have the different numbers of light emitting cellsconnected in series and are connected in reverse parallel to oneanother. Further, a plurality of second arrays are positioned on thesubstrate. The second arrays have the different numbers of lightemitting cells connected in series and are connected in reverse parallelto the first arrays. Accordingly, it is possible to provide an AClight-emitting device, wherein the arrays are sequentially turned on andthen turned off in reverse order under an AC power source so that aflicker effect can be reduced and light emission time can be increased.

In some embodiments, the light emitting device may further comprise aswitching block for controlling light emission of each of the pluralityof first and second arrays depending on a voltage level of an AC powersource. The switching block selectively controls the light emission ofthe arrays depending on the voltage level of the AC power source. Atthis time, one terminal of each of the first and second arrays isconnected in common to a first power source connection terminal, and theother terminals thereof are connected to second power source connectionterminals, respectively. In addition, the switching block may beconnected between the second power source connection terminals and theAC power source to form a current path depending on the voltage level ofthe AC power source, thereby controlling the light emission of thearrays.

In some embodiments, each of the second arrays may have the same numberof light emitting cells as each of the corresponding arrays in the firstarrays. Thus, it is possible to provide a light emitting device havingidentical light output and light emitting spectrum according to a phasechange in an AC power source. Further, an array having the greaternumber of the light emitting cells in the first and second arrays mayhave larger light emitting cells.

In some embodiments, first resistors may be respectively connected inseries to the first arrays. Moreover, second resistors may berespectively connected in series to the second arrays. The first andsecond resistors are employed to prevent an excessive current fromflowing into the first and second arrays.

The first resistors may have different resistance values, and the secondresistors may also have different resistance values. The first andsecond resistors may be respectively connected in series to the firstand second arrays in such a manner that a resistor with a largerresistance value is connected to an array having the smaller number ofthe light emitting cells. Accordingly, an excessive current can beprevented from flowing in an array that has been first turned on.

Meanwhile, the second resistors may have resistance values correspondingto those of the first resistors. Accordingly, it is possible to providea light emitting device having identical light output and light emittingspectrum in relation with a phase change in an AC power source.

Instead of the first and second resistors, a common resistor may beserially connected in common to the first and second arrays.Accordingly, the arrays are sequentially operated due to differences inthe numbers of the light emitting cells. Since the common resistor isconnected to the respective arrays, a process of connecting a resistoris simplified.

The resistors or common resistor may be positioned on the substrate oroutside the substrate. That is, the resistors or common resistor may beformed inside an LED chip, or alternatively may be formed in anadditional resistor device that in turn is connected to the arrays.

Meanwhile, in each of the first and second arrays, light emitting cellswithin an array having the smallest number of the light emitting cellsmay have luminous intensity larger than that of light emitting cellswithin an array having the greatest number of the light emitting cells.Accordingly, it is possible to provide an AC light-emitting device,wherein the arrays are sequentially turned on and then turned off inreverse order under an AC power source so that light emission time canbe increased, and arrays which are initially turned on have largerluminous intensity so that a flicker effect can be reduced.

The light emitting cells within the array having the smallest number ofthe light emitting cells may have roughened surfaces so that they canhave luminous intensity larger than that of the light emitting cellswithin the array having the greatest number of the light emitting cells.The roughened surfaces of the light emitting cells prevent totalreflection due to difference in refraction index, thereby improving theextraction efficiency of light emitted to the outside. Accordingly, theluminous intensity of the light emitting cell is increased.

The light emitting cells with the roughened surfaces may constitutearrays of which the number is ½ of the number of the arrays in each ofthe first and second arrays. In this case, the arrays of the lightemitting cells with the roughened surfaces have the relatively smallernumber of the light emitting cells. Meanwhile, the arrays of the lightemitting cells with the roughened surfaces may be limited to arrayshaving the smallest number of the light emitting cells in each of thefirst and second arrays.

Meanwhile, the light emitting cells within the array having the smallestnumber of the light emitting cells may have inclined side surfaces sothat they can have luminous intensity larger than that of the lightemitting cells within the array having the greatest number of the lightemitting cells. The inclined side surfaces improve light extractionefficiency to increase the luminous intensity of the light emittingcells.

The light emitting cells with the inclined side surfaces may constitutearrays of which the number is ½ of the number of the arrays in each ofthe first and second arrays. In this case, the arrays of the lightemitting cells with the inclined side surfaces are have the relativelysmaller number of the light emitting cells. Meanwhile, the arrays of thelight emitting cells with the inclined side surfaces may be limited toarrays having the smallest number of the light emitting cells in each ofthe first and second arrays.

A further aspect of the present invention provides a light emittingdevice for an AC power operation, the device comprising a bridgerectifier.

The AC light-emitting device according to this aspect includes asubstrate. A plurality of arrays are positioned on the substrate. Theplurality of arrays have different numbers of light emitting cellsconnected in series and are connected in parallel to one another. Inaddition, a bridge rectifier is connected to the plurality of arrays.Two nodes of the bridge rectifier are connected to both end portions ofthe arrays, respectively. Accordingly, it is possible to provide an AClight emitting device, wherein the arrays are driven by means of acurrent rectified by the bridge rectifier, and it is also possible toprovide a light emitting device, wherein the arrays are repeatedlyturned on in sequence and then turned off in reverse order due todifference in the numbers of the light emitting cells.

The bridge rectifier may be positioned on the substrate. Accordingly,the bridge rectifier may be formed together with the light emittingcells. On the contrary, a bridge rectifier may be separately providedand then connected to the arrays.

In some embodiments, a switching block may be connected between one endportions of the arrays and one of the nodes of the bridge rectifier. Theswitching block controls light emission of each of the plurality of thearrays depending on a voltage level of an AC power source.

In some embodiments, resistors may be positioned between the bridgerectifier and the plurality of the arrays and connected in series to theplurality of the arrays, respectively. The resistors prevent anexcessive current from flowing into the arrays.

The resistors may have different resistance values. At this time, theresistors are respectively connected in series to the plurality ofarrays in such a manner that a resistor with a larger resistance valueis connected to an array having the smaller number of the light emittingcells. Accordingly, an excessive current can be prevented from flowingin an array that has been first turned on.

Instead of the plurality of resistors, a common resistor may be seriallyconnected in common to the plurality of arrays. If the common resistoris employed, a process of connecting a resistor can be simplified.

Meanwhile, an array having the greater number of the light emittingcells in the plurality of arrays may have larger light emitting cells.

In some embodiments, in the plurality of arrays, light emitting cellswithin an array having the smallest number of the light emitting cellsmay have luminous intensity larger than that of light emitting cellswithin an array having the greatest number of the light emitting cells.For example, the light emitting cells within the array having thesmallest number of the light emitting cells may have roughened surfacesso that they can have luminous intensity larger than that of the lightemitting cells within the array having the greatest number of the lightemitting cells. Alternatively, the light emitting cells within the arrayhaving the smallest number of the light emitting cells may have inclinedside surfaces so that they can have luminous intensity larger than thatof the light emitting cells within the array having the greatest numberof the light emitting cells.

A still further aspect of the present invention provides an AClight-emitting device comprising a delay phosphor. The AC light-emittingdevice according to this aspect comprises an LED chip having a pluralityof light emitting cells; a transparent member for covering the LED chip;and a delay phosphor excited by light emitted from the light emittingcells to emit light in a visible light range.

Here, the term “delay phosphor” is also referred to as a long afterglowphosphor, and means a phosphor having a long decay time after anexcitation light source is blocked. Here, the decay time is defined as atime taken to reach 10% of an initial value after the excitation lightsource has been blocked. In this embodiment, the delay phosphor may havea decay time of 1 msec or more, preferably about 8 msec or more.Meanwhile, although an upper limit of the decay time of the delayphosphor is not limited to specific values, it may be preferred that itbe not too long according to uses of light emitting devices. Forexample, in case of a light emitting device used for general householdillumination, the decay time of the delay phosphor is preferably a fewminutes or less. The decay time is sufficiently 10 hours or less evenfor interior illumination.

According to this embodiment, when an LED chip emits visible light, thelight emitted from the LED chip and light emitted from the delayphosphor are mixed to increase the light emission time of the lightemitting device, thereby preventing a flicker effect of an AClight-emitting device.

The delay phosphor may be positioned between the LED chip and thetransparent member, or dispersed within the transparent member.

The delay phosphor may be one of red, green and blue phosphors, or acombination thereof.

In addition to the delay phosphor, the AC light-emitting device mayfurther comprise another phosphor excited by light emitted from the LEDchip to emit light in a visible light range. Such a phosphor is employedto provide light emitting devices capable of emitting light havingvarious colors by converting the wavelength of light emitted from theLED chip. For example, in a case where the LED chip emits ultravioletlight, the other phosphors may be red, green and/or blue phosphors toemit white light through mixing with light in a visible light rangeemitted from the delay phosphor. Further, in a case where the LED chipemits blue light, the other phosphors may be a red phosphor and/or agreen phosphor, or a yellow phosphor.

According to the present invention, it is possible to provide a lightemitting device for an AC power operation, wherein a flicker effect canbe prevented or reduced and a time during which light is emitted isincreased corresponding to a phase change in an AC voltage. Further, itis possible to provide a light emitting device, wherein an operation canbe made with a lower current as compared with a prior art, therebyimproving light emitting efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are schematic circuit diagrams illustrating a conventionalAC light-emitting device.

FIG. 3 is a schematic graph illustrating a driving voltage and a currentwith time in the conventional AC light-emitting device.

FIG. 4 is a schematic graph illustrating a driving voltage and a lightemission amount of the conventional AC light-emitting device.

FIG. 5 is a partial sectional view illustrating an AC light-emittingdiode (LED) according to an embodiment of the present invention.

FIG. 6 is a partial sectional view illustrating an AC LED according toanother embodiment of the present invention.

FIG. 7 is a schematic circuit diagram illustrating an AC light-emittingdevice according to a first embodiment of the present invention.

FIG. 8 is a schematic graph illustrating a driving voltage and a currentwith time in the AC light-emitting device according to the firstembodiment of the present invention.

FIG. 9 is a schematic circuit diagram illustrating an AC light-emittingdevice according to a second embodiment of the present invention.

FIG. 10 is a schematic graph illustrating a driving voltage and a lightemission amount of the AC light-emitting device according to the secondembodiment of the present invention.

FIG. 11 is a schematic circuit diagram illustrating an AC light-emittingdevice according to a third embodiment of the present invention.

FIG. 12 is a schematic circuit diagram illustrating a modification ofthe AC light-emitting device according to the third embodiment of thepresent invention.

FIG. 13 is a schematic graph illustrating a driving voltage and a lightemission amount of the AC light-emitting device according to the thirdembodiment of the present invention.

FIG. 14 is a schematic circuit diagram illustrating an AC light-emittingdevice according to a fourth embodiment of the present invention.

FIG. 15 is a schematic circuit diagram illustrating an AC light-emittingdevice according to a fifth embodiment of the present invention.

FIG. 16 is a schematic circuit diagram illustrating an AC light-emittingdevice according to a sixth embodiment of the present invention.

FIGS. 17 and 18 are sectional views illustrating LEDs applicable to theembodiments of the present invention.

FIG. 19 is a sectional view illustrating an AC light-emitting deviceaccording to a seventh embodiment of the present invention.

FIG. 20 is a schematic graph illustrating a light emittingcharacteristic of an AC light-emitting device according to the seventhembodiment of the present invention.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

Hereinafter, preferred embodiments of the present invention will bedescribed in detail with reference to the accompanying drawings. Thefollowing embodiments are provided only for illustrative purposes sothat those skilled in the art can fully understand the spirit of thepresent invention. Therefore, the present invention is not limited tothe following embodiments but may be implemented in other forms. In thedrawings, the widths, lengths, thicknesses and the like of elements areexaggerated for convenience of illustration. Like reference numeralsindicate like elements throughout the specification and drawings.

FIG. 5 is a partial sectional view illustrating a light emitting diode(LED) according to an embodiment of the present invention.

Referring to FIG. 5, light emitting cells 30 spaced apart from oneanother are positioned on a substrate 20. Each of the light emittingcells 30 comprises a first conductive type semiconductor layer 25, asecond conductive type semiconductor layer 29 positioned on a region ofthe first conductive type semiconductor layer 25, and an active layer 27interposed between the first and second conductive type semiconductorlayers. Here, the first and second conductive type semiconductor layers25 and 29 are of an N-type and a P-type, or a P-type and an N-type,respectively.

An electrode 31 may be formed on the second conductive typesemiconductor layer 29. The electrode 31 may be a transparent electrodethrough which light can be transmitted.

The light emitting cells 30 can be formed by forming the respectivesemiconductor layers and an electrode layer on the substrate 20 and thenpatterning them using photolithography and etching processes. Thesubstrate 20 may be a substrate made of Al₂O₃, SiC, ZnO, Si, GaAs, GaP,LiAl₂O₃, BN, AN or GaN, and selected depending on the material of asemiconductor layer to be formed on the substrate 20. If a GaN basedsemiconductor layer is formed, the substrate 20 may be an Al₂O₃ or SiCsubstrate.

A buffer layer 21 may be interposed between the substrate 20 and each ofthe light emitting cells 30. The buffer layer 21 is a layer for reducinga lattice mismatch between the substrate 20 and subsequent layers incrystal growth. For example, the buffer layer 21 may be a GaN or AlNfilm. The N-type semiconductor layer is a layer in which electrons areproduced and may be formed of GaN doped with N-type impurities. TheP-type semiconductor layer is a layer in which holes are produced andmay be formed of AlGaN doped with P-type impurities.

The active layer 27 is a region where predetermined band gaps andquantum wells are made so that electrons and holes can be recombined,and may include an Al_(x)In_(y)Ga_(z)N layer. The wavelength of lightemitted when the electrons and holes are combined varies depending on acompositional ratio of elements constituting the active layer 27. Thus,a compositional ratio is between Al, In and Ga is selected to emit lightwith a required wavelength, e.g., ultraviolet or blue light.

An electrode pad 33 a may be formed on a region of the first conductivetype semiconductor layer 25 and an electrode pad 33 b may be formed onthe electrode 31. Each of the electrode pads 33 a and 33 b can be formedat a desired position using a lift-off technique.

Wires 41 electrically connect adjacent light emitting cells 30 to eachother to form an array having the light emitting cells 30 connected inseries. As shown in this figure, each of the wires 41 connects theelectrode pad 33 a formed on the first conductive type semiconductorlayer 25 of one light emitting cell to the electrode pad 33 b formed onthe electrode 31 of the other light emitting cell. The wires 41 may beformed using an air bridge or step-coverage process.

FIG. 6 is a sectional view illustrating an LED according to anotherembodiment of the present invention. Since the LED of this embodiment ismostly identical with that of FIG. 5, only a difference will bedescribed.

Referring to FIG. 6, a reflective metal layer 51 is formed on eachelectrode 31. The reflective metal layer 51 may be formed as a singlelayer or multiple layers. For example, the reflective metal layer may bemade of Ag, and barrier metal layers for preventing diffusion of Ag maybe formed on and beneath the reflective metal layer, respectively. Thereflective metal layer reflects light generated from the active layer 27toward the substrate 20. Thus, it is preferred that the substrate 20 bea light transmitting substrate. The electrode pad 33 b may be formed onthe reflective metal layer 51, or it may be omitted. Meanwhile, insteadof formation of the reflective metal layer 51, the electrode 31 may beformed as a reflective metal layer.

Meanwhile, a metal bumper 53 is formed on the reflective metal layer 51.The metal bumper 53 is bonded on a submount (not shown) to transfer heatgenerated from a light emitting cell.

According to this embodiment, there may be provided a flip-chip type LEDhaving the metal bumpers 53 thermally contacted with a submount. Such aflip-chip type LED has superior heat-dissipating efficiency and thusprovides high light output.

FIG. 7 is a schematic circuit diagram illustrating an AC light-emittingdevice according to a first embodiment of the present invention, andFIG. 8 is a schematic graph illustrating a driving voltage and a currentwith time in the AC light-emitting device according to the firstembodiment of the present invention.

Referring to FIG. 7, the light emitting device comprises an array oflight emitting cells C₁ to C_(n) connected in series. The array isformed on a single LED chip. A plurality of arrays may be formed on asingle chip and these arrays are connected in reverse parallel to oneanother. Accordingly, the light emitting cells are driven while beingconnected to an AC power source.

As described with reference to FIGS. 5 and 6, each of the light emittingcells comprises an N-type semiconductor layer, a P-type semiconductorlayer and an active layer interposed therebetween. The semiconductorlayers and the active layer may be formed as GaN based compoundsemiconductor layers. The semiconductor layers and the active layer arenot limited to the GaN based compound semiconductor layer but may beformed of a variety of material films using various techniques.

The light emitting cells C₁ to C_(n) are connected in series, and nodesL₁ to L_(n-1) are positioned between light emitting cells. Switchingblocks G₁ to G_(n-1) are connected to the nodes L₁ to L_(n-1),respectively. That is, the switching block G₁ is connected to the nodeL₁ between the light emitting cells C₁ and C₂, and the switching blockG₂ is connected to the node L₂ between the light emitting cells C₂ andC₃. The switching block G_(n-1) is connected to the node L_(n-1) betweenthe light emitting cells C₁ and C_(n) in such a manner.

As an AC voltage V_(ac) is increased in a forward direction, theswitching blocks G₁ to G_(n-1) are sequentially operated to turn on thelight emitting cells C₁ to C_(n) in sequence. Further, if the AC voltagepasses a peak value and is decreased, the switching blocks G₁ to G_(n-1)are sequentially operated again to turn off the light emitting cells C₁to C_(n) in sequence.

Each of the switching blocks G₁ to G_(n-1) is connected to sourceterminal S and ground terminal G of the array of the light emittingcells. Here, if an AC voltage power source is electrically connected toboth ends of the array, a terminal through which a current flows intothe array is referred to as the source terminal S and a terminal throughwhich a current flows out from the array is referred to as the groundterminal G. The AC voltage power source may be connected to the sourceterminal S, and the ground terminal G may be grounded. Alternatively,the source and ground terminals S and G may be connected to both ends ofthe AC voltage power source, respectively. Here, for the sake ofconvenience of illustration, the ground terminal G will be described asbeing grounded. If the ground terminal G is grounded, the switchingblocks G₁ to G_(n-1) may be separately grounded instead of beingconnected to the ground terminal G of the array.

The switching blocks G₁ to G_(n-1) can be driven by means of a voltagedifference between the source and ground terminals S and G, and thisoperation will be described below.

When a voltage V_(ac) of the source terminal S is within a range of(Predetermined Voltage×n) to (Predetermined Voltage×(n+1)), each of theswitching blocks G_(n) shorts the node L_(n) and the ground terminal G.Here, n denotes an ordinal number for a switching block. In this case,the switching blocks G₁ to G_(n-1) bypass a current to the groundterminal G. Meanwhile, if the voltage V_(ac) of the source terminal S is(Predetermined Voltage×(n+1)) or more, each of the switching blocksG_(n) opens the node L_(n) from the ground terminal G. In this case, thecurrents that are bypassed through the switching blocks G₁ to G_(n-1)are cut off. Further, if the voltage V_(ac) of the source terminal S issmaller than (Predetermined Voltage×n), each of the switching blocks G₁to G_(n-1) opens the node L_(n) from the ground terminal G.

The predetermined voltage may be a forward voltage for the lightemitting cells at a reference current. The reference current isdetermined in consideration of the light emitting efficiency of thelight emitting cells. For example, the reference current may bedetermined as a current at which the light emitting efficiency of thelight emitting cells is maximized. Such a current may be 15 to 25 mA andis 20 mA in GaN based semiconductor layers and active layers.

The operation of the light emitting device according to this embodimentwill be described in detail below. Here, a forward voltage V_(f) for alight emitting cell at a reference current will be described as thepredetermined voltage.

If an AC voltage power source is connected to the source terminal S sothat a voltage of the source terminal becomes larger than the forwardvoltage V_(f), the switching block G₁ shorts the node L₁ and the groundterminal G. Accordingly, the reference current begins to flow throughthe light emitting cell C₁ and the switching block G₁, and the lightemitting cell C₁ to is operated to emit light. If the AC voltage V_(ac)further increases, a current exceeding the reference current flowsthrough the light emitting cell C₁.

Subsequently, if the AC voltage reaches 2V_(f), the switching block G₁opens the node L₁ from the ground terminal G to cut off a bypassingcurrent. Meanwhile, the switching block G₂ shorts the node L₂ and theground terminal G. Accordingly, the light emitting cell C₂ is turned on,and the reference current flows into the ground terminal G through thelight emitting cells C₁ and C₂ and the switching block G₂. That is, thecurrent is bypassed from the node L₂ to the ground terminal G by meansof the switching block G₂.

As the AC voltage V_(ac) is increased, the processes in which theswitching blocks G₁ to G_(n-1) are shorted and subsequently opened arerepeated, so that the switching blocks G₁ to G_(n-1) are sequentiallyopened, and thus, the light emitting cells G₁ to G_(n) are sequentiallyturned on.

Meanwhile, if the AC voltage V_(ac) is decreased after a time of T/4 hasbeen elapsed, the opened switching block G_(n-1) is shorted and thelight emitting cell C_(n) is turned off. Subsequently, if the AC voltageV_(ac) is further decreased, the switching block G_(n-1) is opened, andthe opened switching block G_(n-2) (not shown) is shorted to turn offthe light emitting cell C_(n-1). That is, as the AC voltage V_(ac) isdecreased, the processes in which the switching blocks G₁ to G_(n-1) areshorted and subsequently opened are repeated in sequence, and the lightemitting cells are sequentially turned off.

Table 1 below summarizes the turn-on and turn-off operations of thelight emitting cells with time for a ½ cycle.

Time C₁ C₂ C₃ . . . C_(n) Cn⁻¹ 0 off off off . . . off off on off off .. . off off on on off . . . off off on on on . . . off off on on on . .. on off T/4 on on on . . . on on on on on . . . on on on on on . . . onon on on on . . . on off on on on . . . off off on on off . . . off offon off off . . . off off T/2 Off off off . . . off off

Referring to Table 1, the light emitting cells C₁ to C_(n) aresequentially turned on to with the lapse of time, and then turned off inreverse order after a time of T/4 has been elapsed. Under a drivingvoltage V_(ac) at which all light emitting cells are turned on in aprior art, all the light emitting cells according to this embodiment arealso turned on. Additionally, according to this embodiment, some lightemitting cells are turned on even before the driving voltage V_(ac)turns on all the light emitting cells.

Referring to FIG. 8, as the driving voltage V_(ac) is increased, thecurrent flowing through the light emitting cells C₁ to C_(n) has asubstantially constant current value until all the light emitting cellsare turned on. The current substantially corresponds to the referencecurrent. It will be apparent that if the driving voltage V_(ac) isfurther increased even after all the light emitting cells have beenturned on, the current flowing through the light emitting cells areincreased.

Meanwhile, since the light emitting cells are sequentially turned on,some of the light emitting cells emit light even when the drivingvoltage V_(ac) has a small value. Further, since the light emittingcells are sequentially turned off, some of the light emitting cells emitlight even when the driving voltage V_(ac) has a small value. Thus, aneffective time during which the light emitting cells are driven isincreased.

According to this embodiment, as the light emitting cells C₁ to C_(n)are sequentially turned on and off, an effective time during which thelight emitting cells are turned on to emit light is increased as a wholeas compared with the prior art. Accordingly, if the same AC voltagepower source as the prior art is used, light output is improved. Inother words, even though a peak value of the driving voltage V_(ac) isset to be smaller as compared with the prior art, it is possible toprovide the same light output as the prior art. Thus, the light emittingcells can be driven using a low current. Accordingly, the junctiontemperature of the light emitting cells can be lowered, therebyimproving the light emitting efficiency.

The switching blocks G₁ to G_(n-1) are not limited to this embodimentbut may be implemented into a variety of modifications. For example, theswitching blocks may be configured as an identical circuit, and anadditional circuit for sequentially operating the switching blocks maybe added. Further, a circuit may be provided within or separately fromthe switching blocks to prevent an excessive current from flowingthrough operating light emitting cells when a source voltage V_(ac)increases in a certain voltage range. For example, such a circuit mayinclude a constant voltage source such as a zener diode, or a resistor.Accordingly, it can be prevented that an excessive current flows throughlight emitting cells C₁ to C_(n-1) that have been turned on, before anarbitrary light emitting cell C_(n) is turned on.

Meanwhile, at least two arrays of light emitting cells connected inseries may be connected in reverse parallel to each other and switchingblocks may be connected to nodes between the light emitting cells ofeach of the arrays, respectively. Meanwhile, the switching blocks may beconnected in common to the respective arrays. Accordingly, the switchingblocks can allow the light emitting cells within one array to besequentially turned on or off during a ½ cycle and then allow the lightemitting cells within another array to be sequentially turned on and offduring a latter ½ cycle.

FIG. 9 is a schematic circuit diagram illustrating an AC light-emittingdevice according to a second embodiment of the present invention, andFIG. 10 is a schematic graph illustrating a driving voltage and a lightemission amount of the AC light-emitting device according to the secondembodiment of the present invention.

Referring to FIG. 9, the AC light-emitting device according to thisembodiment comprises an LED 200 and a switching block 300. The LED 200comprises a plurality of arrays 101 to 108 each of which has lightemitting cells connected in series. The arrays 101 to 108 are positionedon a single substrate, and each of the arrays has the different numberof the light emitting cells connected in series and thus is driven at adifferent voltage level.

One terminal of each of the arrays 101 to 108 is connected to a firstpower source connection terminal 110, and the other terminals thereofare connected to a plurality of second power source connection terminals121 to 128, respectively. Meanwhile, the switching block 300 isconnected to the plurality of second power source connection terminals121 to 128 of the light emitting diode 200 to form a current pathbetween an external power source 1000 and one of the second power sourceconnection terminals 121 to 128 in accordance with for example, voltagevalues of the external power source 1000. Here, the first power sourceconnection terminal 110 is connected to the external power source 1000.

For example, the LED 200 comprises 8 light emitting cell arrays 101 to108 as shown in FIG. 9, and a plurality of light emitting cells 30 areconnected in series in each of the light emitting cell arrays 101 to 108such that they emit light under a different voltage.

That is, each of the first to fourth arrays 101 and 104 has thedifferent number of the light emitting cells 30 that are connected inseries and connected in a forward direction between the second powersource connection terminals 121 to 124 and the first power sourceconnection terminal 110. Each of the fifth to eighth arrays 105 to 108has the different number of the light emitting cells 30 that areconnected in series and connected in a reverse direction between thesecond power source connection terminals 125 to 128 and the first powersource connection terminal 110. At this time, the terms “forwarddirection” and “reverse direction” are intended to indicate the flowdirection of a current between two terminals. A direction in which acurrent flows from the second power source connection terminals 121 to128 to the first power source connection terminal 110 with respect tothe second power source connection terminals 121 to 128 is referred toas the forward direction, and a direction in which a current flows fromthe first power source connection terminal 110 to the second powersource connection terminals 121 to 128 is referred to as the reversedirection.

Here, the second array 102 has the light emitting cells 30 of which thenumber is larger than that of the first array 101, the third array 103has the light emitting cells 30 of which the number is larger than thatof the second array 102, and the fourth array 104 has the light emittingcells 30 of which the number is larger than that of the third array 103.Further, the sixth array 106 has the light emitting cells 30 of whichthe number is larger than that of the fifth array 105, the seventh array107 has the light emitting cells 30 of which the number is larger thanthat of the sixth array 106, and the eighth array 108 has the lightemitting cells 30 of which the number is larger than the seventh array107. At this time, the first to fourth arrays 101, 102, 103 and 104preferably comprise the light emitting cells 30 of which the numbers areidentical with those of the fifth to eighth arrays 105, 106, 107 and108, respectively.

For example, when a 220V AC power source 1000 is used, the number of thelight emitting cells 30 within each of the first and fifth arrays 101and 105 may be selected such that they emit light in a range where theabsolute value of an AC voltage is 1 to 70V, the number of the lightemitting cells 30 within each of the second and sixth arrays 102 and 106may be selected such that they emit light in a range where the absolutevalue of an AC voltage is 71 to 140V, the number of the light emittingcells 30 within each of the third and seventh arrays 103 and 107 may beselected such that they emit light in a range where the absolute valueof an AC voltage is 141 to 210V, and the number of the light emittingcells 30 within each of the fourth and eighth arrays 104 and 108 may beselected such that they emit light in a range where the absolute valueof an AC voltage is 211 to 280V.

The aforementioned voltage range is only for illustrative purposes, andthe voltage range can be adjusted by changing the number of the lightemitting cells connected in series within light emitting cell arrays.The number of the light emitting cells determines a driving voltage of alight emitting cell array.

As shown in FIG. 9, the switching block 300 according to this embodimentcomprises a first terminal connected to one end of the AC power source1000 and a plurality of second terminals respectively connected to theplurality of second power source connection terminals 121 to 128.Further, the switching block 300 comprises a voltage level determinationunit for determining a voltage level of the AC power source 1000, and aswitch for changing a current path between the AC power source 1000 andthe second power source connection terminals 121 to 128 depending on avoltage level. The switching block 300 is selectively bypassed to thesecond power source connection terminals 121 to 128 in accordance with avoltage applied in a forward or reverse direction.

The switching block 300 forms current paths between the second powersource connection terminals 121 to 128 of the first to eighth arrays 101to 108 and the AC power source 1000 depending on the voltage level ofthe AC power source 1000. For example, if a low forward voltage isapplied, a current path is formed between the AC power source 1000 andthe second power source connection terminal 121 of the first array 101so that the light emitting cells within the first array 101 can emitlight, and if a high reverse voltage is applied, a current path isformed between the AC power source 1000 and the second power sourceconnection terminal 128 of the eighth array 108 so that the lightemitting cells within the eighth array 108 can emit light.

FIG. 10 is a schematic graph illustrating a driving voltage and a lightemission amount of the AC light-emitting device according to thisembodiment. Here, FIG. 10 (a) shows waveforms diagram of an AC powersource, and FIG. 10 (b) shows a light emission amount of the lightemitting device.

Referring to FIG. 10, current paths between the AC current power source1000 and the first to eighth arrays 101 to 108 are changed by theswitching block 300 depending on a voltage level of the AC power source1000. That is, any one of the first to eighth arrays 101 to 108 emitslight at a certain level. Thus, since the light emitting deviceaccording to this embodiment emits light at most voltage levels of theAC power source 1000, an electric power loss and a flicker effect arereduced.

As shown in FIG. 10 (a), the voltage of the AC power source 1000 isperiodically changed with time. When the AC power source is in a forwarddirection, a voltage level of the forward power source is defined asregions A, B, C and D, and an array capable of emitting light isdetermined depending on the regions. That is, if the voltage of the ACpower source 1000 exists within the region A, a current path is formedto the first array 101 by the switching block 300 so that the lightemitting cells within the first array 101 can emit light (See A′ in FIG.10 (b)). Since a small number of the light emitting cells 30 areconnected in series to each other in the first array 101, they areeasily turned on with a small voltage. Further, if the voltage of the ACpower source exists within the region B, a current path is formed to thesecond array 102 by the switching block 300 so that the light emittingcells within the second array 102 can emit light (See B′ in FIG. 10(b)). Furthermore, if the voltage of the AC power source exists withinthe region C, a current path is formed to the third array 103 by theswitching block 300 so that the light emitting cells within the thirdarray 103 can emit light (See C′ in FIG. 10 (b)). In addition, if thevoltage of the AC power source exists within the region D, a currentpath is formed to the fourth array 104 by the switching block 300 sothat the light emitting cells within the fourth array 104 can emit light(See D′ in FIG. 10 (b)).

Meanwhile, when the AC power source is applied in a reverse direction, avoltage level of the reverse power source is defined as regions E, F, Gand H, and an array capable of emitting light is changed depending onthe regions. That is, current paths are selectively formed to the fifthto eighth arrays 105, 106, 107 and 108 by the switching block 300 inaccordance with the voltage level of the AC power source 1000, so thatthe light emitting cells within each of the arrays can sequentially emitlight (See E′, F′, G′ and H′ in FIG. 10 (b)).

FIG. 11 is a schematic circuit diagram illustrating an AC light-emittingdevice according to a third embodiment of the present invention, FIG. 12is a schematic circuit diagram illustrating a modification of the AClight-emitting device according to the third embodiment of the presentinvention, and FIG. 13 is a schematic graph illustrating a drivingvoltage and a light emission amount of the AC light-emitting deviceaccording to the third embodiment of the present invention.

Referring to FIGS. 11 and 12, the AC light-emitting device according tothis embodiment comprises a bridge rectifier 400 for rectifying acurrent from an AC power source 1000, a plurality of arrays 101 to 103each of which has a plurality light emitting cells connected in seriestherein, and a switching block 300 connected to the arrays. The arrays101 to 103 have the different numbers of light emitting cells.

Meanwhile, one terminal of each of the plurality of arrays 101 to 103 isconnected to a first power source connection terminal 110 connected tothe bridge rectifier 400, and the other terminals thereof are connectedto a plurality of second power source connection terminals 121 to 123,respectively. The switching block 300 is connected to the plurality ofsecond power source connection terminals 121 to 123 of the lightemitting device 200 and the bridge rectifier 400. The switching block300 forms current paths between the bridge rectifier 400 and the secondpower source connection terminals 121 to 123 depending on the level of avoltage to be rectified by the bridge rectifier 400.

As shown in FIG. 11, the bridge rectifier 400 may be formed with diodeportions 410 to 440 disposed between the first to fourth nodes Q1 to Q4.That is, anode and cathode terminals of the first diode portion 410 areconnected to the first and second nodes Q1 and Q2, respectively. Anodeand cathode terminals of the second diode portion 420 are connected tothe third and second nodes Q3 and Q2, respectively. Anode and cathodeterminals of the third diode portion 430 are connected to the fourth andthird nodes Q4 and Q3, respectively. Anode and cathode terminals of thefourth diode portion 440 are connected to the fourth and first nodes Q4and Q1, respectively. The first and fourth diode portions 410 to 440 mayhave the same structure as the light emitting cells 30. That is, thefirst and fourth diode portions 410 to 440 may be formed on the samesubstrate while forming the light emitting cells 30. At this time, thefirst and third nodes Q1 and Q3 of the bridge rectifier 400 areconnected to the AC power source 1000, the second node Q2 is connectedto the switching block 300, and the fourth node Q4 is connected to thefirst power source connection terminal 110.

The bridge rectifier 400 may be provided using rectification diodesseparate from the LED 200.

As shown in FIG. 12, the LED 200 may be positioned inside a bridgerectifier. This shows that a bridge rectifier 400 is manufactured usingextra light emitting cells formed on edge portions of an LED chip uponmanufacture of a light emitting diode comprising arrays each of whichhas a plurality of light emitting cells connected in series.

FIG. 13 is a schematic graph illustrating the operation of the AClight-emitting device according to the third embodiment of the presentinvention. Here, FIG. 13 (a) is waveforms diagram of the AC power sourceapplied to the second node Q2 of the bridge rectifier 400, and FIG. 13(b) is a graph illustrating a light emission amount of the lightemitting device.

When an AC voltage is applied, a power source with a form in which areverse voltage is reversed is produced through the rectificationoperation of the bridge rectifier 400, as shown in FIG. 13 (a).Accordingly, only a forward voltage is applied to the switching block300. Thus, the arrays 101, 102 and 103 are connected in parallel to eachother such that the LED 200 can emit light in response to the forwardvoltage.

If the level of a voltage of the AC power source rectified by the bridgerectifier 400 exists in a region A, a current path is formed to thefirst array 101 by the switching block 300 so that the light emittingcells within the first array can emit light. At this time, if theapplied voltage is larger than the sum of threshold voltages of thelight emitting cells 30 connected in series within the first array 101,light emission is started, and a light emission amount is increased asthe applied voltage is increased. Thereafter, a current path is formedto the second array 102 by the switching block 300 if the voltage risesso that the voltage level reaches a region B, and a current path isformed to the third array 103 if the voltage level reaches a region C.

The AC light-emitting device according to this embodiment emits light atalmost all the voltage levels of an AC power source without powerconsumption, and a time during which light is emitted is increased toreduce a flicker effect.

FIG. 14 is a circuit diagram illustrating an AC light-emitting deviceaccording to a fourth embodiment of the present invention. Hereinafter,n represents an integer of 2 or more.

Referring to FIG. 14, the light emitting device has an LED 200 andadditionally, comprises first resistors R1 to Rn and second resistorsR′1 to R′n.

The LED 200 comprises, on a single substrate, a plurality of firstarrays A1 to An and a plurality of second arrays RA1 to RAn, each ofwhich has the light emitting cells (30 in FIG. 5) connected in series.The first arrays A1 to An are connected in parallel to each other, andthe second arrays RA1 to RAn are connected in reverse parallel to thefirst arrays.

The first arrays A1 to An have the different numbers of light emittingcells connected in series. That is, the number of the light emittingcells within the array A1 is smaller than that of the light emittingcells within the array A2, and the number of the light emitting cellswithin the array A2 is smaller than that of the light emitting cellswithin the array An. Further, the second arrays RA1 to RAn have thedifferent numbers of light emitting cells connected in series. That is,the number of the light emitting cells within the array RA1 is smallerthan that of the light emitting cells within the array RA2, and thenumber of the light emitting cells within the array RA2 is smaller thanthat of the light emitting cells within the array RAn.

The second arrays RA1 to RAn may have the light emitting cells of whichthe numbers correspond to those of the first arrays, respectively. Thatis, the array RA1 has the light emitting cells of which the number isidentical with that of the array A1, the array RA2 has the lightemitting cells of which the number is identical with that of the arrayA2, and the array RAn has the light emitting cells of the number isidentical with that of the array An. Thus, the respective second arraysconnected in reverse parallel are paired with the corresponding firstarrays.

One end portions of the first and second arrays may be connected to oneanother on the substrate (20 in FIG. 5). To this end, a bonding pad (notshown) is provided on the substrate 20, and the end portions of thearrays may be connected to the bonding pad through wires. The wires forconnecting the arrays to the bonding pad on the substrate 20 may beformed while forming the wires 41 in FIG. 5. Thus, it is unnecessary toconnect the one end portions of the arrays through bonding wires,resulting in simplification of a wiring process.

Meanwhile, in the first and second arrays A1 to An and RA1 to RAn, anarray having the greater number of the light emitting cells may havelarger light emitting cells 30. That is, the light emitting cells withinthe same array may have the active layers (27 in FIG. 5) withsubstantially same sizes, but the light emitting cells within differentarrays may have the active layers with different sizes. As the activelayer 27 of the light emitting cell becomes large, the bulk resistanceof the light emitting cell 30 is decreased. Accordingly, as a voltage isincreased under a voltage larger than a threshold voltage, the amount ofa current flowing through the light emitting cell 30 is rapidlyincreased.

Meanwhile, the first resistors R1 to Rn are connected in series to thefirst arrays A1 to An, respectively, and the second resistors R′1 to R′nare connected in series to the second arrays RA1 to RAn, respectively.The first and second resistors may be connected to the arrays throughwires, respectively.

The first resistors R1 to Rn may have different resistance values, andthe second resistors R′1 to R′n may also have different resistancevalues. At this time, it is preferred that the first and secondresistors be respectively connected in series to the first and secondarrays in such a manner that a resistor with a larger resistance valueis connected to an array with the smaller number of the light emittingcells 30. That is, the resistor R1 or R′1 with a larger resistance valueis connected in series to the array A1 or RA1 with the smaller number ofthe light emitting cells 30, and the resistor Rn or R′n with a smallerresistance value is connected in series to the array An or RAn with agreater number of the light emitting cells 30.

End portions of the resistors and end portions of the arrays areconnected to both terminals of the AC power source 1000. Here, the ACpower source 1000 may be a general household AC voltage power source.The operation of the AC light-emitting device will be described indetail below.

First, a half cycle in which a positive voltage is applied to theresistors and a negative voltage is applied to one end portion of eachof the arrays opposite to the resistors by means of the AC power sourcewill be described.

As an AC voltage is increased from a zero voltage, the first arrays A1to An biased in a forward direction are turned on. Since the firstarrays A1 to An have the different numbers of the light emitting cells30, they are turned on, starting from the array A1 with the smallernumber of the light emitting cells 30. The array A1 is turned on whenthe AC current exceeds the sum of threshold voltages of the lightemitting cells within the array A1. As the AC voltage is increased, acurrent is increased to emit light. Meanwhile, if the AC voltage isfurther increased and thus the current is increased, a voltage drop dueto the resistor R1 becomes larger. Accordingly, an excessive current isprevented from flowing through the array A1.

If the AC voltage is further increased and thus exceeds the sum of thethreshold voltages of the light emitting cells within the array A2, thearray A2 is turned on to emit light. As such a process is repeated, thearray An is turned on to emit light. That is, the arrays aresequentially turned on, starting from an array with the smaller numberof the light emitting cells 30.

Subsequently, if the AC voltage passes the maximum peak value at T/4 andthen becomes smaller than the sum of the threshold voltages of the lightemitting cells within the array An, the array An is turned off.Thereafter, the AC voltage is further decreased, so that the array A2 isturned off and the array A1 is then turned off. That is, the firstarrays are sequentially turned off, starting from an array with thegreater number of the light emitting cells 30. This is achieved in anorder reverse to the order in which the arrays are turned on.

When the AC voltage becomes zero again, the half cycle is completed. Inthe latter half cycle, the second arrays RA1 to RAn that are biased in aforward direction as the phase of the AC voltage is changed areoperated. The second arrays are operated in the same manner as the firstarrays.

According to this embodiment, there is provided an AC light-emittingdevice driven by means of the AC power source 1000. Further, since theprocesses in which the first and second arrays A1 to An and RA1 to RAnare sequentially turned on and then turned off in the reverse order arerepeated, a flicker effect can be reduced and a time during which thelight emitting device emits light can be increased as compared with aconventional light emitting device.

Meanwhile, by connecting a resistor with a larger resistance value to anarray with the smaller number of the light emitting cells, an excessivecurrent can be prevented from flowing through an array that has beenfirst turned on due to increase in the voltage.

Further, by connecting larger light emitting cells to an array with thegreater number of the light emitting cells, it is possible to rapidlyincrease the amount of a current flowing through arrays that are turnedon later according to increase in the voltage. That is, the bulkresistance of an array with the greater number of the light emittingcells may be smaller than that of an array with the smaller number ofthe light emitting cells. Thus, once an array with the greater number ofthe light emitting cells is turned on, a current flowing through thearray is rapidly increased as the voltage is increased. Accordingly, anexcessive current is prevented from flowing through an array that hasbeen first turned on. Further, since an excessive current can beprevented by adjusting the sizes of light emitting cells, differencesbetween the resistance values of the resistors can be reduced, andconsequently, the resistance values of the resistors can be decreased.The decrease in the resistance values of the resistors causes increasein light output of the light emitting cells.

FIG. 15 is a schematic circuit diagram illustrating an AC light-emittingdevice according to a fifth embodiment of the present invention.

Referring to FIG. 15, the light emitting device has an LED 200 andadditionally, has a common resistor Rt. The LED 200 has components thatare substantially identical with those of the LED 200 described withreference to FIG. 14.

In the LED 200, one end portions of the first and second arrays A1 to Anand RA1 to RAn may be connected to one another on the substrate 20 inFIG. 5, as described with reference to FIG. 14. Additionally, the otherend portions of the first and second arrays may also be connected to oneanother on the substrate 20 in FIG. 5. The other end portions may beconnected to a bonding pad (not shown) provided on the substrate 20through wires, and the wires may be formed while forming the wires 41 ofFIG. 5.

Meanwhile, instead of the first and second resistors R1 to Rn and R′1 toR′n in FIG. 14, the common resistor Rt is serially connected in commonto the first and second arrays A1 to An and RA1 to RAn. At this time,the common resistor Rt may be connected on the bonding pad through awire.

According to this embodiment, since the common resistor Rt is connectedto the first and second arrays, a process of connecting a resistor issimplified as compared with the light emitting device of FIG. 14.

FIG. 16 is a schematic circuit diagram illustrating an AC light-emittingdevice according to a sixth embodiment of the present invention.

Referring to FIG. 16, the light emitting device has an LED 500 and abridge rectifier 350, and may include resistors R1 to Rn.

The LED 500 comprises a plurality of arrays A1 to An each of which haslight emitting cells 30 connected in series on the single substrate 20in FIG. 5. The arrays A1 to An are connected in parallel to one another.

The arrays A1 to An have the different numbers of light emitting cellsconnected in series. That is, the number of the light emitting cellswithin the array A1 is smaller than that of the light emitting cellswithin the array A2, and the number of the light emitting cells withinthe array A2 is smaller than that of the light emitting cells within thearray An.

One end portions of the arrays may be connected to one another on thesubstrate 20 in FIG. 5. To this end, a bonding pad (not shown) isprovided on the substrate 20, and the end portions of the arrays may beconnected to the bonding pad through wires. The wires for connecting thearrays to the bonding pad on the substrate 20 may be formed whileforming the wires 41 in FIG. 5.

Further, in the arrays A1 to An, an array with the greater number of thelight emitting cells may have larger light emitting cells 30.

The bridge rectifier 350 may be configured with light emitting cellsidentical with the light emitting cells 30. Thus, the light emittingcells of the bridge rectifier may be formed while forming the lightemitting cells 30.

One end portions of the arrays A1 to An are connected in common to onenode of the bridge rectifier 350. Such connection can be achieved byconnecting the one node and the bonding pad through a wire.

Meanwhile, the resistors R1 to Rn may be connected in series to thearrays A1 to An, respectively.

The resistors R1 to Rn may have different resistance values. At thistime, it is preferred that the resistors be respectively connected inseries to the arrays in such a manner that a resistor with a largerresistance value is connected to an array with the smaller number of thelight emitting cells 30. That is, the resistor R1 with a largerresistance value is connected in series to the array A1 with the smallernumber of the light emitting cells 30, and the resistor Rn with asmaller resistance value is connected in series to the array An with thegreater number of the light emitting cells 30.

As shown in this figure, one end portions of the resistors R1 to Rn areconnected to another node of the bridge rectifier.

Both terminals of the AC power source 1000 are connected to two othernodes of the bridge rectifier 350, respectively. Here, the AC powersource 1000 may be a general household AC voltage power source. Theoperation of the AC light-emitting device will be described below.

First, when a voltage is applied by the AC power source 1000, a positivevoltage is applied to the resistors through the bridge rectifier 350 anda negative voltage is applied to one end portions of the arrays oppositeto the resistors.

As an AC voltage is increased from a zero voltage, the first arrays A1to An biased in a forward direction are turned on. Since the firstarrays A1 to An have the different numbers of the light emitting cells30, they are turned on, starting from the array A1 with the smallernumber of the light emitting cells 30. The array A1 is turned on whenthe AC current exceeds the sum of threshold voltages of the lightemitting cells within the array A1. As the AC voltage is increased, acurrent is increased to emit light. Meanwhile, if the AC voltage isfurther increased and thus the current is increased, a voltage drop dueto the resistor R1 becomes larger. Accordingly, an excessive current isprevented from flowing through the array A1.

If the AC voltage is further increased and thus exceeds the sum of thethreshold voltages of the light emitting cells within the array A2, thearray A2 is turned on to emit light. As such a process is repeated, thearray An is turned on to emit light. That is, the arrays aresequentially turned on, starting from an array with the smaller numberof the light emitting cells 30.

Subsequently, if the AC voltage passes the maximum peak value at T/4 andthen becomes smaller than the sum of the threshold voltages of the lightemitting cells within the array An, the array An is turned off.Thereafter, the AC voltage is further decreased, so that the array A2 isturned off and the array A1 is then turned off. That is, the firstarrays are sequentially turned off, starting from an array with thegreater number of the light emitting cells 30. This is achieved in anorder reverse to the order in which the arrays are turned on.

When the AC voltage becomes zero again, the half cycle is completed.Even in the latter half cycle, the arrays A1 to An are operated in thesame manner by the bridge rectifier 350.

According to this embodiment, there is provided an AC light-emittingdevice driven by means of the AC power source 1000. Further, since theprocesses in which the arrays A1 to An are sequentially turned on andthen turned off in the reverse order are repeated, a flicker effect canbe reduced and a time during which the light emitting device emits lightcan be increased as compared with a conventional light emitting device.

Although the resistors R1 to Rn are respectively connected in series tothe arrays A1 to An in this embodiment, the common resistor Rt insteadof the resistors R1 to Rn may be connected as shown in FIG. 15. In thiscase, the end portions of the arrays A1 to An may be connected to oneanother on the substrate 20 in FIG. 5.

Meanwhile, in the aforementioned embodiments, the number of the lightemitting cells or a difference in size between the light emitting cellsgenerates a difference in relative luminous intensity between thearrays. That is, since an array that is first turned on has the smallernumber of the light emitting cells, overall luminous intensity thereofis relatively weak. Moreover, since an array that is finally turned onhas the greater number of the light emitting cells, luminous intensitythereof is relatively strong. Such a difference may be enlarged due todifferences in the sizes of the light emitting cells and the resistors.Although there is an advantage in that the arrays are early turned on,the luminous intensity of the arrays initially turned on is relativelysmall. Thus, it may not greatly contribute to reduction in the flickereffect. Thus, it is necessary to increase the luminous intensity of thearrays initially turned on. To this end, the structures of the lightemitting cells within the respective arrays may be constructeddifferently. FIGS. 17 and 18 are sectional views illustrating LEDs inwhich the structures of light emitting cells within some arrays aremodified to increase the luminous intensity of the light emitting cellswithin the arrays.

Referring FIG. 17, the LED according to this embodiment is substantiallyidentical with that described with reference to FIG. 5, and the lightemitting cell arrays may be arranged in the same manner as the first tosixth embodiments. However, the light emitting cell in this embodimentis different from that of FIG. 5 in that a top surface of the lightemitting cell, e.g., the surface of a second conductive typesemiconductor layer 29 a, is roughened.

Light emitting cells each of which has the second conductive typesemiconductor layer 29 a with the roughened surface may construct thearrays in the first and sixth embodiments of the present invention,which have been previously described above. Further, in the first andsixth embodiments, arrays of which the number is ½ or less of the numberof entire arrays may be constructed of the light emitting cells havingthe second conductive type semiconductor layers 29 a with the roughenedsurfaces. For example, arrays of which the number is ½ or less of thenumber of the first arrays and arrays of which the number is ½ or lessof the number of the second arrays in FIGS. 14 and 15 may be constructedof the light emitting cells having the second conductive typesemiconductor layers 29 a with the roughened surfaces; and arrays ofwhich the number is ½ or less of the number of the arrays in FIG. 16 maybe constructed of the light emitting cells having the second conductivetype semiconductor layers 29 a with the roughened surfaces. At thistime, the light emitting cells having the second conductive typesemiconductor layers 29 a with the roughened surfaces constitute arrayshaving the smaller number of the light emitting cells, and arrays havingthe greater number of the light emitting cells are constructed of lightemitting cells having flat surfaces.

The second conductive type semiconductor layer 29 a with the roughenedsurface may be formed by sequentially forming a first conductive typesemiconductor layer 25, an active layer 27 and the second conductivetype semiconductor layer, and etching a region of the second conductivetype semiconductor layer using a photoelectrochemical etching technique.Meanwhile, the roughened surface may be formed by forming a metallicthin film, e.g., with a thickness of 10 to 500 Å, in a region of thesecond conductive type semiconductor layer and performing heat treatmentand etching of the metallic thin film.

Thereafter, the array(s) of the light emitting cells each of which hasthe second conductive type semiconductor layer 29 a with the roughenedsurface is formed by etching the region of the semiconductor layers toform the light emitting cells and electrically connecting the lightemitting cells to one another.

Meanwhile, an electrode pad 33 b is formed on the second conductive typesemiconductor layer 29 a with the roughened surface, and the electrode31 in FIG. 5 may also be formed thereon.

The arrays each of which has the smaller number of the light emittingcells, e.g., the arrays A1 and RA1 of FIGS. 14 and 15 or the array A1 ofFIG. 16, are constructed of the light emitting cells having theroughened surfaces, so that the luminous intensity of these arrays canbe increased. Thus, since the luminous intensity of the arrays A1 andRA1 that are initially turned on when an AC voltage is applied theretois large, a flicker effect can be further reduced.

The roughened surface may be formed on a bottom surface of the substrateof FIG. 6. Alternatively, the substrate 20 may be separated in FIG. 6and the roughened surface may be formed on a bottom surface of the firstconductive type semiconductor layer 25.

Referring to FIG. 18, light emitting cells in this embodiment aresubstantially identical with those described with reference to FIG. 5,and light emitting cell arrays may be arranged in the same manner asdescribed in the first to sixth embodiments. However, in thisembodiment, each of the light emitting cells is formed to have inclinedside surfaces. The light emitting cells each of which has the inclinedside surfaces constitute the arrays (A1 and RA1 in FIGS. 14 to 17) thatare initially turned on in the same manner as the light emitting cellshaving the roughened surfaces, and may constitute arrays of which thenumber is ½ of the number of the first and second arrays in FIGS. 14 to17.

The inclined side surfaces may be formed by reflowing a photoresistpattern, and then etching the semiconductor layers using the photoresistpattern as an etching mask, in the separation process of the lightemitting cells after the formation of semiconductor layers.Additionally, the inclined side surfaces may be formed by performingreflow of the photoresist pattern and etching the second conductive typesemiconductor layer and the active layer when a region of the firstconductive type semiconductor layer 25 is exposed.

The inclined side surfaces of the light emitting cells can reduce alight loss due to total reflection, thereby improving luminous intensityof the light emitting cells. Thus, by forming arrays, which areinitially turned on, out of such light emitting cells, a flicker effectcan be reduced as described with reference to FIG. 17.

FIG. 19 is a sectional view illustrating an AC light-emitting device 1according to a seventh embodiment of the present invention.

Referring to FIG. 19, the light emitting device 1 comprises an LED chip3. The LED chip 3 has a plurality of light emitting cells connected inseries. Each of the light emitting cells may be an Al_(x)In_(y)Ga_(z)Nbased compound semiconductor capable of emitting ultraviolet or bluelight. The structures of the LED chip 3 and the light emitting cells areidentical with those described with reference to FIG. 5 or 6. The lightemitting cells are connected in series and constitute an array. The LEDchip 3 may comprise two arrays connected in reverse parallel, or maycomprise a bridge rectifier, so that it can be driven by means of an ACpower source.

The LED chip 3 is electrically connected to an external power sourcethrough lead terminals (not shown). To this end, the LED chip 3 may havetwo bonding pads (not shown) used for connection to the lead terminals.The bonding pads are connected to the lead terminals through bondingwires (not shown). On the contrary, the LED chip 3 may be flip-bonded toa submount substrate (not shown) and then electrically connected to thelead terminals through the submount substrate.

Meanwhile, the LED chip 3 may be positioned within a reflection cup 9.The reflection cup 9 reflects light emitted from the LED chip 3 in arange of desired viewing angles to increase luminance within a certainrange of viewing angles. Thus, the reflection cup 9 has a predeterminedinclined surface depending on a required viewing angle.

Meanwhile, phosphors 7 are positioned over the LED chip 3 and thenexcited by light emitted from the light emitting cells to emit light ina visible light range. The phosphors 7 include a delay phosphor. Thedelay phosphor may have a decay time of 1 msec or more, preferably,about 8 msec or more. Meanwhile, an upper limit of the decay time of thedelay phosphor can be selected depending on uses of light emittingdevices and may be, but not specifically limited to, 10 hours or less.Particularly, in case of a light emitting device used for generalhousehold illumination, the decay time of the delay phosphor ispreferably a few minutes or less.

The delay phosphor may be a silicate, an aluminate, a sulfide phosphoror the like disclosed in U.S. Pat. Nos. 5,770,111, 5,839,718, 5,885,483,6,093,346, 6,267,911 and the like. For example, the delay phosphor maybe (Zn,Cd)S:Cu, SrAl₂O₄:Eu,Dy, (Ca,Sr)S:Bi, ZnSiO₄:Eu,(Sr,Zn,Eu,Pb,Dy)O.(Al,Bi)₂O₃, m(Sr,Ba)O.n(Mg,M)O.2(Si,Ge)O₂:Eu,Ln(wherein, 1.5≦m≦3.5; 0.5≦n≦1.5; M is at least one element selected fromthe group consisting of Be, Zn and Cd; and Ln is at least one elementselected from the group consisting of Sc, Y, La, Ce, Pr, Nd, Sm, Gd, Th,Dy, Ho, Er, Tm, Yb, Lu, B, Al, Ga, In, Tl, Sb, Bi, As, P, Sn, Pb, Ti,Zr, Hf, V, Nb, Ta, Mo, W, Cr and Mn), or the like.

The delay phosphor is excited by light emitted from the LED chip 3 toemit light in the visible light range, e.g., red, green and/or bluelight. Accordingly, it is possible to provide a light emitting devicecapable of emitting light having various colors by mixing light emittedfrom the LED chip 3 with light emitted from the delay phosphor, and toprovide a light emitting device capable of emitting white light.

Meanwhile, the phosphors 7 may include other phosphors capable ofemitting light in the visible light range while being excited by lightfrom the LED chip 3, e.g., red, green and/or blue phosphors, or yellowphosphors, in addition to the delay phosphor. For example, the otherphosphors may be YAG:Ce based phosphors, orthosilicate based phosphors,or sulfide phosphors.

The delay phosphors and the other phosphors are selected such that thelight emitting device emits light having a desired color. In case of awhite light emitting device, the delay phosphor and the other phosphorsmay be formed of various combinations of phosphors such that lightobtained by mixture of light emitted from the LED chip 3 with convertedlight becomes white light. Further, a combination of the delay phosphorand the other phosphors may be selected in consideration of flickereffect prevention, light emitting efficiency, a color rendering index,and the like.

Meanwhile, a transparent member 5 may cover the LED chip 3. Thetransparent member 5 may be a coating layer or a molded member formedusing a mold. The transparent member 5 covers the LED chip 3 to protectthe LED chip 3 from external environment such as moisture or an externalforce. For example, the transparent member 5 may be made of epoxy orsilicone resin. In a case where the LED chip 3 is positioned within thereflection cup 9, the transparent member 5 may be positioned within thereflection cup 9 as shown in this figure.

The phosphors 7 may be positioned between the transparent member 5 andthe LED chip 3. In this case, the transparent member 5 is formed afterthe phosphors 7 have been applied to the LED chip 3. On the contrary,the phosphors 7 may be dispersed within the transparent member 5 asshown in this figure. There have been known a variety of techniques ofdispersing the phosphors 7 within the transparent member 5. For example,the transparent member 5 may be formed by performing transfer moldingusing mixed powder with phosphors and resin powder mixed therein, or bydispersing phosphors within liquid resin and curing the liquid resin.

FIG. 20 is a schematic graph illustrating a light emittingcharacteristic of an AC light-emitting device according to an embodimentof the present invention. Here, a dotted line (a) is a curveschematically illustrating a light emitting characteristic of aconventional AC light-emitting device, and a solid line (b) is a curveschematically illustrating a light emitting characteristic of a lightemitting device according to the embodiment of the present invention.

Referring to FIG. 20, the conventional light emitting device that doesnot employ a delay phosphor periodically repeats on-off by means ofapplication of an AC voltage thereto. Assuming that the period of an ACpower source is T, two arrays each of which has light emitting cellsconnected in series are alternately operated once during the period T.Thus, the light emitting device emits light with a period of T/2 asrepresented by the dotted line (a). Meanwhile, if the AC voltage doesnot exceed a threshold voltage of the light emitting cells connected inseries, the light emitting cells are not operated. Thus, the lightemitting cells remain in a turned-off state for a certain time, i.e.,for a time during which the AC voltage is smaller than the thresholdvoltage thereof, between times during which the light emitting cells areoperated. Accordingly, a flicker effect may appear in the conventionallight emitting device due to gaps between times during which the lightemitting cells are operated.

On the other hand, since the light emitting device according to theembodiment of the present invention employs a delay phosphor, light isemitted even while the light emitting cells remain in the turned-offstate as represented by the solid line (b). Thus, although there is achange in luminous intensity, a time during which light is not emittedbecomes shorter, and the light emitting device continuously emits lightif the decay time of the delay phosphor is long.

When a general household AC power source applies a voltage with afrequency of about 60 Hz, one cycle of the power source is about 16.7msec and a half cycle is 8 msec. Thus, while the light emitting deviceis in operation, a time during which all the light emitting cells areturned off is smaller than 8 msec. Accordingly, if the decay time of thedelay phosphor is 1 msec or more, a flicker effect can be sufficientlyreduced. Particularly, if the decay time of the delay phosphor issimilar to a time during which all the light emitting cells are turnedoff, the light emitting device can continuously emit light.

What is claimed is:
 1. A light-emitting device, comprising: a firstpower source connection terminal and a second power source connectionterminal; and a plurality of first arrays of GaN-based light emittingcells, each first array comprising a plurality of serially connectedlight emitting cells, the first arrays being connected in parallel toone another between the first power source connection terminal and thesecond power source connection terminal, and each first array beingconfigured to emit light under a different voltage level from the otherfirst arrays, wherein the GaN-based light emitting cells aremonolithically formed on a substrate, and wherein an N-typesemiconductor layer of one light emitting cell is electrically connectedto a P-type semiconductor layer of an adjacent light emitting cell. 2.The light-emitting device of claim 1, wherein the first arrays havedifferent numbers of light emitting cells from each other.
 3. Thelight-emitting device of claim 2, further comprising: a plurality offirst resistors connected in series to the first arrays, wherein eachfirst resistor is respectively connected to one of the first arrays. 4.The light-emitting device of claim 3, wherein the first resistors havedifferent resistance values from each other, and the first resistors arerespectively connected in series to the first arrays in such a mannerthat one of the first resistors comprising a larger resistance valuerelative to the other first resistors is connected to a first arraycomprising a smaller number of light emitting cells relative to theother first arrays, and one of the first resistors comprising a smallerresistance value relative to the other first resistors is connected to afirst array comprising a larger number of light emitting cells relativeto the other first arrays.
 5. The light-emitting device of claim 1,further comprising: a plurality of second arrays of light emittingcells, each second array comprising a plurality of serially connectedlight emitting cells, the second arrays being connected in reverseparallel to the first arrays.
 6. The light-emitting device of claim 1,wherein the substrate consists of Al₂O₃ or SiC.
 7. A light-emittingdevice, comprising: a bridge rectifier; and a plurality of lightemitting cell arrays, each array comprising a plurality of seriallyconnected GaN-based light emitting cells, the arrays being connected inparallel to one another between two nodes of the bridge rectifier, andeach array is configured to emit light under a different voltage levelfrom the other arrays, wherein the GaN-based light emitting cells aremonolithically formed on a substrate, wherein the bridge rectifiercomprises a bridge circuit in which at least one diode is disposedbetween a first node and a second node, at least one diode is disposedbetween the second node and a third node, at least one diode is disposedbetween the third node and a fourth node, and at least one diode isdisposed between the fourth node and the first node of the bridgecircuit, respectively, and wherein each diode comprises a light emittingcell.
 8. The light-emitting device of claim 7, wherein each arraycomprises a different number of light emitting cells than the otherarrays.
 9. The light-emitting device of claim 8, further comprising: aplurality of resistors connected in series to the arrays, wherein eachresistor is respectively connected to one of the arrays.
 10. Thelight-emitting device of claim 9, wherein the resistors have differentresistance values from each other, and wherein the resistors arerespectively connected in series to the arrays in such a manner that oneof the resistors comprising a larger resistance value relative to theother resistors is connected to an array comprising a smaller number oflight emitting cells relative to the other arrays, and one of theresistors with a smaller resistance value relative to the otherresistors is connected to an array comprising a larger number of lightemitting cells relative to the other arrays.
 11. The light-emittingdevice of claim 7, wherein the bridge rectifier and the plurality oflight emitting cell arrays are formed on a substrate.
 12. Thelight-emitting device of claim 11, wherein the substrate consists ofAl₂O₃ or SiC.